A) SONY and the Estate has released a statement saying that it's him, with the latter naming 6 people who are very respected in the music world confirming it's authenticity, as well as Dorian Holley, MJ's Vocal Director.
B) As I previously stated, I believe it to be somewhat psychological. We were given the idea of a fake MJ song, something we hadn't thought of in an official product. While we brushed it off, when one of the songs in question came online, our natural instinct is to listen intently to try and listen to any abnormalities. Given that the song is practically a demo brought to "completion", MJ's vocals are not what we expect from an album song. Because of this, many people were confused, and other people believe it to be fake altogether. It can be assumed that the Cascio tracks didn't have much development by MJ himself in terms of vocals, so the Estate has done the best that they could considering that these tracks are some of his last recorded work.
That's the best I can answer, yet somehow I got the feeling someone would use my questions against my argument... Oh, well... I brought it upon myself.